The present invention is generally related to the inspection of nuclear reactors and more particularly to an apparatus for inspecting the upper internals containing the control rod guide structures in a nuclear reactor.
In nuclear reactors, control rod assemblies are used to control the reactivity of the reactor. The upper internals and control rod guide structures house and guide the control rod assemblies as they are raised from or lowered into the core. The guide structure is generally formed by one or more horizontally positioned plates above the reactor core. A centrally located vertical aperture and radial channels through each plate house and guide the control rods and control rod assemblies.
The known spider form of the assemblies passes through the slot-like channels as the rods are raised and lowered. The channels through the plate extend radially relative to the aperture. Each channel is provided with at least one cutout section that forms a C-shape or split tube shape in the channel. During normal reactor operation the control rod assemblies are withdrawn from the core. In this position, a relatively short length of the control rods extends into the core. The hydraulic flow of coolant through the upper internals causes the control rods to vibrate against the horizontal plate of the guide structure. This vibration causes wear of the control rods and horizontal plate, especially at the cutout sections of the channels in the plate. Substantial wear of the guide plate can have an adverse effect on the normal operation of the control rods. Thus, it becomes important to know the extent of wear in the guide structure since proper reactor control is directly related to proper control rod movement. See, for example, U.S. Pat. Nos. 4,728,488, 4,863,678 and 4,902,468.
Inspection of this plate presents special problems. The radial channels and central aperture are complex in shape. Difficulty in accessing the plates, radioactive contamination and high radiation levels, and the need for precise inspection measurements preclude a close visual inspection.